Failed debit.

Honestly Nedbank is the worst. I haven’t had any problems with them till recently, when they failed to debit month end. I called them, several times being transferred from one department to another with no solution. Even went to the branch still no help. Now they are telling me my account is in errears, while they are the ones who failed to debit. One of the consultants advised on they system that, I will make a cash deposit claiming my bank is at fault, how? because they have been debiting for the past 3 years with no errors. This is affecting my credit score and if this is not fixed, should I seek legal action?